Killer whales (Orcinus orca) in Iceland show weak genetic structure among diverse isotopic signatures and observed movement patterns

Abstract

Local adaption through ecological niche specialization can lead to genetic structure between and within populations. In the Northeast Pacific, killer whales (Orcinus orca) of the same population have uniform specialized diets that are non‐overlapping with other sympatric, genetically divergent, and socially isolated killer whale ecotypes. However, killer whales in Iceland show intrapopulation variation of isotopic niches and observed movement patterns: some individuals appear to specialize on herring and follow it year‐round while others feed upon herring only seasonally or opportunistically. We investigated genetic differentiation among Icelandic killer whales with different isotopic signatures and observed movement patterns. This information is key for management and conservation purposes but also for better understanding how niche specialization drives genetic differentiation. Photo‐identified individuals (N = 61) were genotyped for 22 microsatellites and a 611 bp portion of the mitochondrial DNA (mtDNA) control region. Photo‐identification of individuals allowed linkage of genetic data to existing data on individual isotopic niche, observed movement patterns, and social associations. Population subdivision into three genetic units was supported by a discriminant analysis of principal components (DAPC). Genetic clustering corresponded to the distribution of isotopic signatures, mtDNA haplotypes, and observed movement patterns, but genetic units were not socially segregated. Genetic differentiation was weak (F ST < 0.1), suggesting ongoing gene flow or recent separation of the genetic units. Our results show that killer whales in Iceland are not as genetically differentiated, ecologically discrete, or socially isolated as the Northeast Pacific prey‐specialized killer whales. If any process of ecological divergence and niche specialization is taking place among killer whales in Iceland, it is likely at a very early stage and has not led to the patterns observed in the Northeast Pacific.

The relevance of genetic structure in ecotype designation and conservation management

“…Yet, some populations with high rates of mixing (those that exhibit fission-fusion) may lack these intuitive social structures (Tavares et al, 2018). In such cases, distinguishing the relevant scale at which to interpret genetic data becomes more difficult, requiring detailed ecological knowledge of the species and the population under study.…”

“…Genetic data can be incorporated with ecological information such as stable isotope analyses, and the geographical scale of habitat use and movements, to improve the designation of meaningful conservation units (Nykänen et al, 2019 ; Thiemann et al, 2008 ). This approach can also help resolve cases where genetic units are not socially segregated but comprise individuals with dietary and behavioural differences, to show whether a process of niche specialization and emergence of ecotypes might have started and require further study (Tavares et al, 2018 ). Investigations into ecologically significant genetic structure may be especially problematic for species that occur at low densities with limited N e , as this parameter is considered vital for evolutionary resilience (Sgrò et al, 2011 ) but can be hard to determine.…”

“…However, we must also be aware that as analytical power increases, we augment magnification until at last we observe highly localized structures (Carstens et al, 2013 ), which can include basic social and family structure in wild species. Yet, some populations with high rates of mixing (those that exhibit fission–fusion) may lack these intuitive social structures (Tavares et al, 2018 ). In such cases, distinguishing the relevant scale at which to interpret genetic data becomes more difficult, requiring detailed ecological knowledge of the species and the population under study.…”

“…Genomic analyses of killer whale ecotypes have reported adaptive variation associated with several traits linked to environmental variation, including cold adaptation, dietary variation and reproductive function (Foote et al, 2016 ). Forthcoming research combining genomics, satellite tracking, dietary analyses and other data can offer further resolution on killer whale evolutionary history (Whitehead, 2017 ), including ecotypes that have so far proved more difficult to study (COSEWIC, 2008 ; Lefort et al, 2020 ) and populations where patterns of possible ecological divergence are currently less clear (de Bruyn et al, 2013 ; Tavares et al, 2018 ). Killer whale ecotypes may now represent incipient (sub)species (Morin et al, 2010 ; Moura et al, 2015 ; Whitehead, 2017 ).…”

“…Hence, the cultural transfer of knowledge could hasten the development of ecotypes (Riesch et al, 2012 ) but might also constrain adaptation to rapidly changing environmental conditions, with consequences for their long‐term conservation status and management (COSEWIC, 2008 ). Some potential killer whale ecotypes may be more difficult to categorize depending on their stage in the developmental process (Riesch et al, 2012 ; Tavares et al, 2018 ). Given considerable variation in life and demographic histories across species, the development of (incipient) ecotypes in this and other taxa could therefore occur at different spatiotemporal scales.…”